The invention is directed to a continuous-acting press formed by multi-layered press bands with each press band running over a pair of return rollers. Each of the band sets is formed of a plurality of mutually contacting press bands with the upper and lower band sets being spaced apart and having mutually facing surfaces forming a space for receiving a material web to be pressed.
To produce decorative laminate materials, chipboards, fiberboards, plywood boards, electrical laminates and the like, it is necessary to subject the material to high pressure and a high temperature so that the resin becomes completely cured and the products are of the required quality. It is, therefore, necessary for the press being used to be capable of exerting the required pressure and of transmitting the necessary heat.
Continuous and intermittent presses are used for the production of the above-mentioned materials. To increase the output of the intermittent-acting single-daylight presses, multi-daylight presses are used, where the material is piled in layers on individual platform sheets and the latter are then placed each into a daylight of the multi-daylight press. During the subsequent pressing of the material, it is necessary for the temperature and pressure cycles of the press to take into consideration the outermost press sets, whereby at the edges approximately 40 to 50 mm of the material cures without the application of pressure and has to be trimmed off afterwards, representing a waste of 5 to 10%. Furthermore, an intermittent-acting press can be integrated into a continuously operating production installation only with a great outlay for apparatus. Therefore, a continuous-acting press is also desirable, whereby the finished product is obtained in a continuous sheet or board, which is also desirable from the standpoint of low waste.
In such continuous-acting presses, two continuous steel bands, which transmit the operating pressure and the amount of heat to the material necessary to cure the resin and simultaneously draw the material through the press, are passed each over a drive drum and a return drum so that a reaction zone is formed between the mutually facing band surfaces. Customarily, pressure is transmitted mechanically by rollers rolling on the press bands or hydraulically by means of chambers filled with fluid pressure media and sealed from the exterior. The steel band must not exceed a certain thickness in view of the admissible flexural stresses of the continuous bands which occur during the deviation. However, the pressure which can be transmitted to the material is limited by the tensile strength and thickness of the steel band, which greatly restricts the capacity of the continuous press, and renders its use for the production of technical laminates, such as electrical laminates, difficult.
Because the pressure to be transmitted is directly proportional to the thickness of the steel band, it is known to pile a plurality of bands one upon another to form a set (see German Offenlegungsschrift 1,939,784) to increase the thickness while simultaneously maintaining the flexural strength, and to transmit the increased pressure through such a multi-layered band set. In this way the possibility of increasing the pressure as much as required for the material is advantageously created. The thermal capacity is thereby simultaneously increased in proportion to the sum of the individual thicknesses of all the bands, when the process heat is transmitted to the bands outside the reaction zone and fed into the reaction zone stored in the bands.
It is, however, a disadvantage in the use of a multi-layered band set that during the operation of the press the individual press bands can wander laterally relative to one another. It is, therefore, proposed in German Pat. No. 2,735,142 that the individual press bands of a multi-layered band set exhibit, in mutually contacting band surfaces, grooves in which centering means are inserted or fastened. However, it is to be considered a disadvantage in this case that the misaligning forces at right angles to the travel direction of the multi-layered band set act upon two centering means of small cross-section inserted along the band edge on the left and right, and that there is, therefore, a possibility of these centering means being destroyed at high pressures.
The reaction zone formed by the two opposite multi-layered band sets is closed upwardly and downwardly by the multi-layered band set, forwardly in the travel direction of the material web by cured resin and rearwardly by unmolten resin. However, it is open along the sides, with the result that surplus resin can escape laterally. The laminate, therefore, becomes cured, without the application of pressure in the marginal region and the required thickness tolerance cannot be realized at that location, so this so-called press edge has to be cut off after the laminate is completed.